1. Field of the Invention
The present invention relates to a torque sensor for magnetically detecting a torque transmitted to a rotating shaft without direct. contact with the rotating shaft.
2. Description of the Prior Art
Conventional torque sensors of the type utilizing magnetostriction are classified into two groups. A first group of torque sensors include a rotating shaft made of a magnetic alloy, such as a ferro-alloy, having a magnetostrictive property, and a second group of torque sensors include a rotating shaft having on its outer peripheral surface a magnetic alloy layer having a soft magnetic property and magnetostrictive property. In either group of conventional torque sensors, the magnetostrictive magnetic alloy is magnetically anisotropic in different directions slanting at angles of +45.degree. and -45.degree. , respectively, with respect to a longitudinal axis of the shaft. Two coils are disposed around the magnetostrictive magnetic alloy for detecting a magnetic property of the magnetostrictive magnetic alloy. The coils are received end to end within a tubular container which is made of an alloy of soft magnetic property and provided for purposes of excluding the influence of a disturbance magnetic field and completing a closed magnetic circuit. The tubular container is hereinafter referred to as "magnetic yoke".
With this construction, when a torque is transmitted to the rotating shaft of the torque sensor, an outer peripheral surface of the shaft is strained or otherwise deformed. In this instance, if the torque exerted on the shaft is clockwise, a portion of the magnetostrictive magnetic alloy which is magnetically anisotropic in the direction of +45.degree. increases its magnetic permeability, while a portion of the magnetostrictive magnetic ally which is magnetically anisotropic in the direction of -45.degree. decreases its magnetic permeability. This relation in magnetic permeability is reversed when the torque exerted on the shaft is counterclockwise. Changes in magnetic permeability of the magnetostrictive magnetic alloy are detected in terms of changes in self-inductance of the two coils, and the difference in self-inductance between the two coils is measured by a differential detector whereby the direction and magnitude of the torque can be detected (see, Japanese Patent Laid-open Publication No. 59-77326). The two coils have the same inductance so that in a theoretical sense, no differential output is produced from the torque sensor when a torque is not exerted on the shaft. However, when the magnetic yoke is secured by screws to a structure for attaching the torque sensor to the structure, the torque sensor produces an apparent differential output due to tightening forces applied to the magnetic yoke. In addition, when the environmental temperature changes, a stress is created due to a difference in thermal expansion coefficient between the structure and the magnetic yoke. The stress thus created results in generation of an apparent differential output from the torque sensor.
It has been proved that the apparent differential output is caused by magnetostriction of the magnetic yoke. It is obvious that the apparent differential output can be reduced by using a magnetic yoke which is made of a soft magnetic property material, such as 78% Ni--Mo--Cu--Fe alloy, having a small magnetostrictive property. However, such a material is expensive and, hence, the torque sensor is costly to manufacture.